1. Field of the Invention
The present invention relates to a flash device for illuminating an object of shooting, a control method and a control apparatus for the flash device, and an image pickup apparatus, such as an electronic still camera, which uses the flash device.
2. Description of Related Art
In a case where a CCD of the all-pixel-reading type is employed as an image sensor in taking a picture by using a flash device, one still image can be recorded by emitting light once during a period of accumulating electric charge for one field. A continuous shooting also can be accomplished by continuously recording still images with one field used as the unit of shooting and by emitting light a plurality of times in synchronism with the continuous recording.
On the other hand, if a still image is to be recorded to have an excellent vertical resolution with a CCD of the field reading type employed as an image sensor, such recording is possible by using video signals for two continuous fields. For this purpose, light emission must be made twice for the two continuous field in synchronism with each field. In addition to that, it is necessary to control the amount of light emission in such a way as to make the amount of light emission for one field equal to the amount of light emission for the other field.
A conventional method for controlling the amount of light emission is described below with reference to FIGS. 5 and 6.
FIG. 5 is a diagram for explaining changes taking place in voltage of a main capacitor of a flash device when the flash device is allowed to emit light once in synchronism with a field of recording by an electronic still camera. In FIG. 5, reference numeral 401 denotes a waveform of light emission by the flash device, reference numeral 402 denotes a charging voltage of the main capacitor obtained before light emission, reference numeral 403 denotes a voltage which corresponds to the amount of light emission, reference numeral 404 denotes a voltage at which light emission becomes possible (hereinafter referred to as a light-emission enabling voltage), and reference numeral 405 denotes a charging voltage of the main capacitor obtained after light emission. A maximum amount of light emission is set as an upper limit of light emission amount for control to be accomplished over the amount of light emission by considering the charging voltage of the main capacitor. A light emission amount at which an apposite exposure can be obtained is compared with the maximum light emission amount, and an amount of light emission to be made by the flash device is controlled so as not to exceed the maximum light emission amount according to the result of comparison.
In a case where the light emission is required only once, the maximum light emission amount is set at a light emission amount corresponding to the charging voltage of the main capacitor obtained before light emission. At that time, if a light emission amount required for an apposite exposure is found to be larger than the maximum light emission amount, an amount of light emission is corrected so as to be set to the maximum light emission amount. Then, light emission is possible so long as the charging voltage 402 before light emission is larger than the light-emission enabling voltage 404.
FIG. 6 shows the control of the amount of light emission and the change in voltage of the main capacitor in a case where light emission is made continuously for two fields by a system similar to the system in FIG. 5. In the case of FIG. 6, the change of voltage is assumed to take place either in a case where a continuous shooting operation is performed in synchronism with fields by an electronic still camera using a CCD of the all-pixel reading type or where a still picture is taken by an electronic still camera using a CCD of the field reading type.
In FIG. 6, reference numeral 501 denotes a waveform of light emission for a first field, reference numeral 502 denotes a waveform of light emission for a second field, reference numeral 503 denotes a charging voltage obtained before light emission for the first field, reference numeral 504 denotes a voltage which corresponds to the amount of light emission for the first field, reference numeral 505 denotes a charging voltage before light emission for the second field, reference numeral 506 denotes a voltage which corresponds to the amount of light emission for the second field, reference numeral 507 denotes a charging voltage obtained after light emission for the second field, and reference numeral 508 denotes a light-emission enabling voltage. The voltage 504 corresponding to the amount of light emission for the first field and the voltage 506 corresponding to the amount of light emission for the second field are equal to each other.
In the case of the continuous light emission, the maximum light emission amount is set at a value obtained by dividing the light emission amount which corresponds to the charging voltage 503 before light emission by the number of times of light emission. Then, like in the case of the single (one-time) light emission, a light emission amount required for an apposite exposure is compared with the maximum light emission amount, and an amount of light emission to be made by the flash device is then controlled so as not to exceed the maximum light emission amount according to the result of comparison.
After the light emission for the first field, the voltage of the main capacitor of the flash device obtained before the light emission for the first field drops from the charging voltage 503 as much as the voltage 504 corresponding to the light emission amount, thereby becoming the charging voltage 505. Since the light emission amount for one field is arranged to be equal to the light emission amount for the other field, the charging voltage drops also after the light emission for the second field as much as the voltage 506 corresponding to the light emission amount for the second field. When the continuous light emission comes to an end, the charging voltage of the main capacitor thus changes to the charging voltage 507.
However, in cases where the charging voltage of the main capacitor before light emission is sufficiently large and the light emission amount required for an apposite exposure is extremely large, or where light emission is to be made immediately after the charging voltage of the main capacitor comes to exceed a light-emission enabling voltage, in carrying out the continuous light emission under the conventional flash light emission amount control mentioned above, the light emission for the first field in a waveform 601 causes the charging voltage of the main capacitor to drop to a charging voltage 605 which is lower than a light-emission enabling voltage 604, as shown in FIG. 7. In such a case, further light emission is inhibited by a flash light emission control circuit even if a light emission trigger pulse is outputted for the second field. The continuous light emission then hardly can be carried out for a predetermined number of times.
Under such a condition, in the case of the system using a CCD of the field reading type, the luminance of an image recorded for the second field greatly differs from that of an image recorded for the first field, so that flicker occurs in a recorded still image, thereby deteriorating the image quality to an excessive degree. This has presented a serious problem.
In accordance with one aspect of the invention, there are provided a flash device in which, in causing a light emitting portion to make light emission n times (n being a predetermined integer not less than 2), an amount of light emission of the light emitting portion for each of the n times is so controlled as to be set to an amount of light emission corresponding to a voltage level which is one (nxe2x88x921)-th of a result obtained by subtracting a light-emission enabling reference voltage level of the light emitting portion from a charging voltage level of a capacitor which supplies light-emission energy to the light emitting portion, so that the light emission for the predetermined n times can be securely performed, a control method for the flash device, an image pickup apparatus using the flash device, and a control apparatus for the flash device.